This invention relates to a bar code which utilizes penetrability of infrared rays and a method of reading the bar code. A semiconductor wafer thus theretofore been adapted to record the bar code on its surface and to then read it thereby establishing a product control.
FIG. 1 is a view showing the general manner in which a bar code 2 formed on a semiconductor wafer 1 is read. According to this manner, the bar code 2 recorded on the lower portion of the semiconductor wafer 1 in a disc form is optically read by a reflector type bar code reader 10 positioned so as to face the semiconductor wafer 1. A semiconductor laser 11 which serves as a light source, is disposed within the bar code reader 10 and emits laser beams LK to then irradiate it through an optical lens 12 on a rotating mirror 12. The laser beams are scanned by the rotation of the rotating mirror 13 and irradiated on the bar code 2 on the semiconductor wafer 1. Reflected beams FK from the bar code 2 are incident to the interior of the bar code reader 10 and are received or sensed through a lens 14 by a photodetector 15 so that a bar code data BD which has been converted to an electrical signal is outputted.
In the aforementioned reading method, the surface of the semiconductor wafer 1 is generally mirror processed or finished, and the wafer 1 has been chemically processed in a step of a manufacturing process and is always subjected to variations in its surface profile. This is also the case with the surface profile of the bar code 2 if the latter is directly marked on the wafer. Under such circumstances, the aforementioned reflector type bar code reader 10 is likely to show a lower S/N ratio of the photodetector 15 due to the influence of the reflection of beam on the surface of the semiconductor wafer 1 and variation of the surface profile of the bar code 2 resulting in increase of a rate of misreading of the bar code 2. This will be of no use for the desired end, and its solution has been desired.